Embryogenesis in higher plants is a critical stage of the plant life cycle in which the primary organs are established. Embryo development can be separated into two main phases: the early phase in which the primary body organization of the embryo is laid down and the late phase which involves maturation, desiccation and dormancy. In the early phase, the symmetry of the embryo changes from radial to bilateral, giving rise to a hypocotyl with a shoot meristem surrounded by the two cotyledonary primordia at the apical pole and a root meristem at the basal pole. In the late phase, during maturation the embryo achieves its maximum size and the seed accumulates storage proteins and lipids. Maturation is ended by the desiccation stage in which the seed water content decreases rapidly and the embryo passes into metabolic quiescent state. Dormancy ends with seed germination, and development continues from the shoot and the root meristem regions.
The precise regulatory mechanisms which control cell and organ differentiation during the initial phase of embryogenesis are largely unknown. The plant hormone abscisic acid (ABA) is thought to play a role during late embryogenesis, mainly in the maturation stage by inhibiting germination during embryogenesis (Black, M. (1991). In Abscisic Acid: Physiology and Biochemistry, W. J. Davies and H. G. Jones, eds. (Oxford: Bios Scientific Publishers Ltd.), pp. 99-124) Koornneef, M., and Karssen, C. M. (1994). In Arabidopsis, E. M. Meyerowitz and C. R. Sommerville, eds. (Cold Spring Harbor: Cold Spring Harbor Laboratory Press), pp. 313-334). Mutations which effect seed development and are ABA insensitive have been identified in Arabidopsis and maize. The ABA insensitive (abi3) mutant of Arabidopsis and the viviparous1 (vp1) mutant of maize are detected mainly during late embryogenesis (McCarty, et al., (1989) Plant Cell 1, 523-532 and Parcy et al., (1994) Plant Cell 6, 1567-1582). Both the VP1 gene and the ABI3 genes have been isolated and were found to share conserved regions (Giraudat, J. (1995) Current Opinion in Cell Biology 7:232-238 and McCarty, D. R. (1995). Annu. Rev. Plant Physiol. Plant Mol. Biol. 46:71-93). The VP1 gene has been shown to function as a transcription activator (McCarty, et al., (1991) Cell 66:895-906). It has been suggested that ABI3 has a similar function.
Another class of embryo defective mutants involves three genes: LEAFY COTYLEDON1 and 2 (LEC1, LEC2) and FUSCA3 (FUS3). These genes are thought to play a central role in late embryogenesis (Baumlein, et al. (1994) Plant J. 6:379-387; Meinke, D. W. (1992) Science 258:1647-1650; Meinke et al., Plant Cell 6:1049-1064; West et al., (1994) Plant Cell 6:1731-1745). Like the abi3 mutant, leafy cotyledon-type mutants are defective in late embryogenesis. In these mutants, seed morphology is altered, the shoot meristem is activated early, storage proteins are lacking and developing cotyledons accumulate anthocyanin. As with abi3 mutants, they are desiccation intolerant and therefore die during late embryogenesis. Nevertheless, the immature mutants embryos can be rescued to give rise to mature and fertile plants. However, unlike abi3 when the immature mutants germinate they exhibit trichomes on the adaxial surface of the cotyledon. Trichomes are normally present only on leaves, stems and sepals, not cotyledons. Therefore, it is thought that the leafy cotyledon type genes have a role in specifying cotyledon identity during embryo development.
Among the above mutants, the lec1mutant exhibits the most extreme phenotype during embryogenesis. For example, the maturation and postgermination programs are active simultaneously in the lec1 mutant (West et al., 1994), suggesting a critical role for LEC1 in gene regulation during late embryogenesis.
In spite of the recent progress in defining the genetic control of embryo development, further progress is required in the identification and analysis of genes expressed specifically in the embryo and seed. Characterization of such genes would allow for the genetic engineering plants with a variety of desirable traits. For instance, modulation of the expression of genes which control embryo development may be used to alter traits such as accumulation of storage proteins in leaves and cotyledons. Alternatively, promoters from embryo or seed-specific genes can be used to direct expression of desirable heterologous genes to the embryo or seed. The present invention addresses these and other needs.